iridoids and strictosidine

Secologanin, a secoiridoid glucoside, is a pivotal terpenoid intermediate in the biosynthesis of biologically active monoterpenoid indole alkaloids such as reserpine, ajmaline, and vinblastine which are biosynthesized via strictosidine, an alkaloidal glucoside, formed from secologanin and tryptamine. In secologanin biosynthesis, the oxidative cleavage process of loganin to secologanin and the hydroxylation of 7-deoxyloganin to loganin have remained unknown enzymologically and mechanistically. Cornoside is a unique glucoside with 4-hydroxy-2,5-cyclohexadien-1-one (benzoquinol) ring and is widespread in families such as Cornaceae, Oleaceae, and Scrophulariaceae but its biosynthesis, especially the oxidative process, remain to be investigated. Shikonin is a red naphthazarin pigment derived from the roots of Lithospermum erythorhizon and produced biotechnologically by cell cultures. Its biosynthesis including the production regulation mechanism has been investigated in detail. However, the naphthazarin ring formation process, probably starting with the hydroxylation of the side chain of geranylhydroquinone, a key intermediate at the late stage of shikonin biosynthesis, remained unknown. In the present review, cytochrome P450 monooxygenases involved in the biosyntheses of three structurally and biosynthetically interesting compounds, secologanin, cornoside, and shikonin, a described together with the results of previous and recent biosynthetic studies. The biosyntheses of related compounds are also discussed.

Topics: Cyclohexanones; Cytochrome P-450 Enzyme System; Glucosides; Indole Alkaloids; Iridoid Glucosides; Iridoids; Naphthoquinones; Oxidation-Reduction; Plants; Secologanin Tryptamine Alkaloids; Tryptamines; Vinca Alkaloids

The first phytochemical examination of extracts from leaves and stem bark of Palicourea luxurians (Rusby) Borhidi yielded two undescribed and one known alstrostine derivative together with the oxindole alkaloid javaniside as well as with 5α-carboxystrictosidine. Additionally, five iridoids and four secologanin derived isolation artifacts have been isolated. Lack of strictosidine and its follow-up metabolization products suggested that the Pictet-Spenglerase in P. luxurians does barely or not catalyze the formation of strictosidine. Against this background the biosynthesis of javaniside and 5α-carboxystrictosidine is discussed with regard to possible reaction mechanisms. Similarly, P. luxurians used an independent biosynthetic pathway to produce alstrostine type structures from secologanin and tryptamine in a 2:1 ratio. The structure of isoalstrostine A, which was isolated for the first time, allowed the refinement of a previously reported pathway to the alstrostine type carbon skeleton as well as to some follow-up metabolization products. In spite of various biosynthetic pathways incorporating secologanin to gain different types of tryptophan- and tryptamine-iridoid alkaloids, P. luxurians accumulates this compound as well a couple of further metabolized iridoids deriving from loganin and secologanin.

Topics: Alkaloids; Iridoid Glucosides; Iridoids; Rubiaceae; Secologanin Tryptamine Alkaloids; Tryptamines; Tryptophan; Vinca Alkaloids

The monoterpene indole alkaloids (MIAs) are a valuable family of chemicals that include the anticancer drugs vinblastine and vincristine. These compounds are of global significance-appearing on the World Health Organization's list of model essential medicines-but remain exorbitantly priced due to low in planta levels. Chemical synthesis and genetic manipulation of MIA producing plants such as Catharanthus roseus have so far failed to find a solution to this problem. Synthetic biology holds a potential answer, by building the pathway into more tractable organisms such as Saccharomyces cerevisiae. Recent work has taken the first steps in this direction by producing small amounts of the intermediate strictosidine in yeast. In order to help improve on these titers, we aimed to optimize the early biosynthetic steps of the MIA pathway to the metabolite nepetalactol. We combined a number of strategies to create a base strain producing 11.4 mg/L of the precursor geraniol. We also show production of the critical intermediate 10-hydroxygeraniol and demonstrate nepetalactol production in vitro. Lastly we demonstrate that activity of the iridoid synthase toward the intermediates geraniol and 10-hydroxygeraniol results in the synthesis of the nonproductive intermediates citronellol and 10-hydroxycitronellol. This discovery has serious implications for the reconstruction of the MIA in heterologous organisms.

Topics: Acyclic Monoterpenes; Bridged Bicyclo Compounds, Heterocyclic; Catharanthus; Indole Alkaloids; Iridoids; Monoterpenes; Saccharomyces cerevisiae; Terpenes; Vinca Alkaloids

Strictosidine synthase (STR1) catalyzes the stereoselective formation of 3alpha(S)-strictosidine from tryptamine and secologanin. Strictosidine is the key intermediate in the biosynthesis of 2,000 plant monoterpenoid indole alkaloids, and it is a key precursor of enzyme-mediated synthesis of alkaloids. An improved expression system is described which leads to optimized His(6)-STR1 synthesis in Escherichia coli. Optimal production of STR1 was achieved by determining the impact of co-expression of chaperones pG-Tf2 and pG-LJE8. The amount and activity of STR1 was doubled in the presence of chaperone pG-Tf2 alone. His(6)-STR1 immobilized on Ni-NTA can be used for enzymatic synthesis of strictosidines on a preparative scale. With the newly co-expressed His(6)-STR1, novel 3alpha(S)-12-azastrictosidine was obtained by enzymatic catalysis of 7-azatryptamine and secologanin. The results obtained are of significant importance for application to chemo-enzymatic approaches leading to diversification of alkaloids with novel improved structures.

Topics: Alkaloids; Biocatalysis; Carbon-Nitrogen Lyases; Catharanthus; Histidine; Iridoid Glucosides; Iridoids; Molecular Chaperones; Oligopeptides; Recombinant Fusion Proteins; Tryptamines; Vinca Alkaloids

A spectrophotometric assay for strictosidine synthase is described. Strictosidine is extracted with ethyl acetate and, where high substrate concentrations are used, the organic extract is washed with dilute ammonia to remove coextracted secologanin; after evaporation of the solvent, the residue is heated with 5 M H2SO4 for 45 min and the A348 value is measured. Strictosidine production is calculated from the response of similarly treated standards. A minimum production of 10-25 nmol of strictosidine may be determined. The assay is demonstrated using extracts of cultured Cinchona ledgeriana cells.

Topics: Carbon-Nitrogen Lyases; Cinchona; Indole Alkaloids; Iridoid Glucosides; Iridoids; Plants, Medicinal; Pyrans; Spectrophotometry, Ultraviolet; Transferases; Tryptamines; Vinca Alkaloids